Comparing to other methods, combustion synthesis method is new for the production of ceramic materials. It utilizes self-propagating combustion reaction between the reactants to produce ceramic products. It has the advantages of high production rate, energy efficiency, simple process, and ease for mass production. However, in view of commercial production, combustion synthesis method still needs some improvements.
The followings are the prior arts utilizing combustion synthesis method for the production of aluminum nitride.
(1) Japanese patent No.63-274,605
With appropriate ratio, prepare powder mixture of aluminum, aluminum nitride, and one of the followings: calcium carbonate (CaCO3), calcium nitrate (Ca(NO3)2), yttrium oxide (Y2O3), barium carbonate (BaCO3), barium nitrate (Ba(NO3)2), yttrium nitrate (Y(NO3)3), cerium oxide (CeO2), and yttrium oxalate (Y2(C2O4)2.8H2O). Press the mixture into appropriate shape and place it in the environment filled with N2 gas up to 50 atm. aluminum nitrate (AlN) powder forms after the combustion reaction is ignited by electrical heating plates.
(2) Japanese patent No.64-76,906
With appropriate ratio, prepare powder mixture of aluminum and aluminum nitride in a refractory container having porous walls and place the container in liquid nitrogen. Ignite the combustion reaction with electrical heating wires. aluminum nitride powder is therefore formed.
(3) Japanese patent No.64-76,905
With appropriate ratio, prepare mixture of aluminum and sodium azide or other solid-state nitrogen containing compounds such as potassium azide (KN3) and barium azide (Ba3N2) powders in a refractory container. With ignition agent above the powder, the container is placed in a reactor with electrical heater and filled with nitrogen less than 10 Kg/cm2. The process starts with heating the reactants with the electrical heater and igniting the ignition agent with electrical heating wire. The combustion reaction is then taken place to form aluminum nitride powder.
(4) ROC patent No.71,873 and U.S. Pat. No. 5,460,794
Prepare mixture of aluminum powder and solid-state nitrogen containing compounds. Press the mixture into shape, cover the mixture with ignition agent, and place it in a chamber filled with nitrogen. The process starts by igniting the ignition agent. The reaction is then taken place to form aluminum nitride powder.
(5) ROC patent No.67,194 and U.S. Pat. No. 5,453,407
Prepare mixture of aluminum powder and solid-state nitrogen containing compounds. Ammonium halide is added into the mixture. Press the mixture into shape, cover the mixture with ignition agent, and place it in a chamber filled with nitrogen. The process starts with ignition of the ignition agent. The reaction is then taken place to form aluminum nitride powder.
(6) U.S. patent application Ser. No. 08/853,576 (filed on May 9, 1997), now U.S. Pat. No. 5,846,508
Prepare mixture of aluminum powder and ammonium halide as reactants. Press the mixture into a compact or place it in a refractory container having an opening end or having porous walls, and place the compact or the container in a chamber filled with nitrogen. The reaction is ignited by heating the reactant mixture to form aluminum nitride powder.
There are two key problems concerning the above mentioned combustion synthesis method for the production of aluminum nitride powder:
(1) How to supply sufficient nitrogen needed;
(2) How to make the reaction complete.
According to 1991.M. Costantino and C. Firpo in J. Mater. Res volumn 6 page 2397, if nitrogen gas is used as nitrogen source, the pressure must reach at least 1,000 atm to start the reaction. According to the application of Japanese patent No.63-274,605, the pressure being used, which is 50atm, is also comparably high. High pressure condition will increase the cost on equipment and operation. Operational complexity and danger are both enhanced.
If liquid nitrogen is used as nitrogen source, as in Japanese patent No.64-76906, high pressure is not necessary. But the low temperature which is required by liquid nitrogen again will increase the cost on equipment and operation. Operational complexity and danger are both enhanced.
If solid-state nitrogen containing compounds are used as nitrogen source, as in Japanese patent No.64-76,905, ROC patent No.71,873 and No.67,194, high pressure is not necessary. But the solid-state nitrogen source needs to thermally decompose easily in order to make the combustion reaction self-propagated. Meanwhile, the reaction needs appropriate arrangement to thermally decompose the solid-state nitrogen source and to make the nitrogen released thereafter react with aluminum powder quickly. Otherwise, problems like pressure accumulation or nitrogen leakage will block the reaction.
The present invention and the filed application of U.S. Ser. No. 08/853,576 have some technical difference. In the application of U.S. Ser. No. 08/853,576, aluminum powder and ammonium halide are used as reactants. In the present invention, aluminum powder and compounds containing NHx or halogens which can be decomposed or vaporized below the melting point of aluminum are used. Therefore, there is a significant difference in the reactants.
In accordance with the present invention, method for production of aluminum nitride powder can be explained by two parts:
(1) Reactant mixture is prepared from aluminum powder and a compound, containing NHx(such as amino group (NH2), ammonia (NH2), ammonium ion (NH4), N2H4, N2H6 ect.) or halogens, which can be thermally decomposed or vaporized.
Reactants discovered in the present invention are aluminum powder and a compound which contains NHx or halogens (such as urea (CO(NH2)2), NH2CO2NH4, ammonium carbonate ((NH4)2CO3), NH4HCO3,HCOONH4, N2H4HCl, N2H4HBr, N2H4.2HCl, aluminum chloride (AlCl3), aluminum bromide (AlBr3), etc.) and can be thermally decomposed or vaporized below the melting point of aluminum . These two reactants are mixed at an appropriate ratio and then pressed into a compact with an appropriate shape (such as cylinder). These two reactants, after being mixed at an appropriate ratio, may also be placed in a refractory container which has an opening at one end or has porous walls (referred to as reactant compact herein after). In making reactant compacts, certain amount of dilutant (such as aluminum nitride powder) can be added.
(2) Ignition of reactant compact takes place by heating at low nitrogen pressure
The above mentioned reactant compact is then placed in a reaction chamber filled with nitrogen. By appropriately heating the reactant compact at one end(such as by tungsten wire, graphite plates, or laser etc., any kind of heating method which can heat one end of the reactant compact up to 1000xc2x0 C.-1600xc2x0 C. can be adopted), the combustion synthesis reaction is ignited and aluminum nitride powder with high quality is produced. The nitrogen pressure required in the reaction is about 0.9-6 atm. (The reaction can also be conducted at higher pressures, but that won""t be necessary.)
The main difference between the present invention and traditional combustion synthesis method for production of aluminum nitride lies in (1) Reactant compact is prepared from aluminum powder and a compound which contains NHx or halogens and can be thermally decomposed or vaporized below the melting point of aluminum. (2) Ignition of reactant compact takes place by heating at low nitrogen pressure.
In accordance with one Example of the present invention, compounds which are introduced in the present invention contain NHx or halogens and can be thermally decomposed or vaporized below the melting point of aluminum. Common examples are urea (CO(NH2)2), NH2CO2NH4, ammonium carbonate ((NH4)2CO3), NH4HCO3, HCOONH4, N2H4HCl, N2H4HBr, N2H4.2HCl, aluminum chloride (AlCl3), aluminum bromide (AlBr3), etc. The above examples are only some of the appropriate compounds. Urea (CO(NH2)2) is the most suitable compound. The weight ratio between urea and aluminum powder can be 0.02-1.5:1 while 0.2:1 is the best ratio.
In accordance with the other Examples of the present invention, among the above mentioned compounds which can be thermally decomposed or vaporized below the melting point of aluminum, urea is the most suitable. The amount of urea can be 0.02-1.5 times the weight of aluminum powder, i.e. the weight ratio between urea and aluminum powder can be 0.02-1.5:1. But 0.2:1 is the best ratio. When pressed into a reactant compact, pressures applied can be 8-50 Kg/cm2 while 10-15 Kg/cm2 is preferred. Appropriate containers are refractory containers having an opening at one end or having porous walls. Refractory containers having an opening at one end can be made from graphite, aluminum nitride, Boron nitride, aluminum oxide, zirconium, or other ceramic materials. Refractory containers having porous walls can be made from graphite, aluminum nitride, Boron nitride, aluminum oxide, zirconium, or other ceramic materials. Heating in nitrogen environment can proceed by applying current to tungsten wire, graphite plates, tungsten plates, graphite resistor which is close to reactant compact. Heating can also proceed by laser irradiation.